<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    #nav {
      /*方案一：增加父级高度*/
      /*height: 300px;*/
      border: 1px solid green;
    }

    /* not(#|class)过滤 */
    div:not(#nav):not(.clear) {
      width: 100px;
      height: 100px;
      border: 1px solid red;
      float: left;
    }

    .clear {
      clear: both;
    }
  </style>
</head>
<body>
<div id="nav">
  <div>div1</div>
  <div>div2</div>
  <div>div3</div>
  <div class="clear"></div>
</div>
</body>
</html>